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Measuring Seeing, The Differential Image Motion Monitor (DIMM)

Measuring Seeing, The Differential Image Motion Monitor (DIMM). Marc Sarazin (European Southern Observatory). List of Themes How to find the ideal site...and keep it good?. Optical Propagation through Turbulence Mechanical and Thermal Index of Refraction

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Measuring Seeing, The Differential Image Motion Monitor (DIMM)

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  1. Measuring Seeing, The Differential Image Motion Monitor (DIMM) Marc Sarazin (European Southern Observatory)

  2. List of ThemesHow to find the ideal site...and keep it good? • Optical Propagation through Turbulence • Mechanical and Thermal • Index of Refraction • Signature on ground based observations • Correction methods • Integral Monitoring Techniques • Seeing Monitoring • Scintillation Monitoring • Profiling Techniques • Microthermal Sensors • Scintillation Ranging • Modelling Techniques Zanjan, Iran

  3. Why Differential Image Motion? • The tracking errors are automatically subtracted • The wind has no effect on the measurements • The telescope optical quality is not important (nevertheless circular images are required, i.e. no coma allowed) • Easy to implement with state of the art amateur astronomer detectors • The DIMM gives two statistical estimates of the same variable Zanjan, Iran

  4. Optical PropagationThe Signature of Atmospheric Turbulence Seeing: (radian, ^-0.2) Fried parameter: ( meter, ^6/5) Zanjan, Iran

  5. DIMM Principle • Two images of the same star are created on a CCD, corresponding to light having traveled through two parallel columns in the atmosphere Zanjan, Iran

  6. DIMM Principle • The variance of the image motion through a circular aperture of diameter D depends on the seeing as: • The variance of the differential image motion through circular apertures of diameter D, separated by d is: Zanjan, Iran

  7. DIMM Principle The final estimate of the seeing is the average of both parallel and perpendicular motions Zanjan, Iran

  8. DIMM Principle Error Budget for a 10% accuracy goal • The instrumental noise (sampling, centroiding) is measured in the lab on fixed sources. The constant part can be subtracted out, the noise is the remaining variance, about +/- 0.002 pixel^2, or 5% relative error at 0.2” seeing. The plate scale is calibrated on double stars of known separation • The measurement noise might increase if the signal to noise ratio is too low: images with low SNR due to scintillation have to be rejected. • The statistical noise is inversely proportional to the square root of the number of samples in the time series. The relative error on the seeing is about 6% for 200 exposures. • The temporal under sampling due to too long exposure time: no way to correct for it because the velocity of the tilt is unknown. Interlacing two exposure times is the best way to control. • The very bad seeing (>2”) is over estimated because the stellar image breaks into speckles Zanjan, Iran

  9. DIMM Precursor A visual DIMM was used in the 60’s for site selection purposes in Chile and in Uzbekistan (photo: Maidanak Observatory). See: J. Stock and G. Keller, 1960, in Stars and Stellar System, Vol. 1, Chicago University Press Zanjan, Iran

  10. Portable DIMM Operation Preparing for nighttime measurements on the high chilean sites (5200m) in the vicinity of the ALMA project Source: Cornell Atacama project http://astrosun.tn.cornell.edu/atacama Zanjan, Iran

  11. Portable DIMM Operation Alignment of C11 telescope mount on a high chilean site (5200m) in the vicinity of the ALMA project • Pixel size=0.7” • Pupil Diameter=9cm • Pupil Separation=12cm • Exposure Time=10/20ms • 50 frames/mn Photo credit: P. Recabarren, Observatory of Cordoba, Argentina Zanjan, Iran

  12. Portable DIMM Operation 1m high platform and daytime protection of the portable DIMM on the high chilean sites (5200m) in the vicinity of the ALMA project Source: Cornell Atacama project http://astrosun.tn.cornell.edu/atacama Zanjan, Iran

  13. Portable DIMM Operation 5m high tower and daytime protection of the portable DIMM at the observatory of Maidanak, Uzbekistan The telescope stands in free air circulation to prevent build-up of local thermal pockets Zanjan, Iran

  14. Automated DIMM Operation Daytime protection of the automated DIMM at the VLT Observatory The enclosure control is linked to the meteorological station (closes when wind>18m/s, Rh>80%) Zanjan, Iran

  15. Automated DIMM Operation 35cm Telescope for the automated DIMM at the VLT Observatory • Pixel size=0.7” • Pupil Diameter=11cm • Pupil Separation=20cm • Exposure Time=5ms • 600 frames/mn Zanjan, Iran

  16. Automated DIMM Operation The seeing is updated every minute for zenith observation at 0.5 micron wavelength The accuracy is better than 10% above 0.2” The natural atmospheric noise is about 10% of the seeing Zanjan, Iran

  17. Automated DIMM Operation The system automatically switches to another star in case of clouds The seeing is independent of cloudiness (although sometimes pretty good with high cirrus clouds) Aperture photometry alows to monitor the sky variability Zanjan, Iran

  18. Automated DIMM Operation Aperture photometry on ca 5000 DIMM short exposures allows to monitor the flux variability, equivalent to the extinction variability (June 2000 statistics below) The threshold for photometric sky is between 1% and 2% relative flux rms Zanjan, Iran

  19. DIMM Seeing vs. VLT Image Quality DIMM converts image motion into large telescope seeing with the assumption of an infinite outer scale of the turbulence. UT images turned out about 10% better than predicted by DIMM, confirming the finite character of the outer scale. Comparison of DIMM seeing (Y axis), with FORS Science Verification (SV) Image Quality (X axis) as processed by the SV team, corrected for zenith and 500nm. Zanjan, Iran

  20. Corrected DIMM Seeing vs. VLT Image Quality DIMM converts image motion into large telescope seeing with the assumption of an infinite outer scale of the turbulence. UT images turned out about 10% better than predicted by DIMM, confirming the finite character of the outer scale. Correcting for that effect is possible by removing from the DIMM the share of the tilt of an 8m aperture. Comparison of DIMM seeing (Y axis) after correction for outer scale, with FORS Science Verification (SV) Image Quality (X axis) as processed by the SV team, corrected for zenith and 500nm. Zanjan, Iran

  21. Corrected DIMM Seeing vs. VLT Image Quality DIMM converts image motion into large telescope seeing with the assumption of an infinite outer scale of the turbulence. UT images turned out about 10% better than predicted by DIMM, confirming the finite character of the outer scale. Correcting for that effect is possible by removing from the DIMM the share of the tilt of an 8m aperture. Comparison of DIMM seeing (Y axis) after correction for outer scale, with UT1 Science Verification (SV) Image Quality (X axis) as processed by the SV team from Test Camera long exposures, corrected for zenith and at 500nm. Zanjan, Iran

  22. DIMM Seeing vs. Large Telescope Image Quality DIMM converts image motion into large telescope seeing with the assumption of an infinite outer scale of the turbulence. Assuming that the outer scale larger than the telescope aperture, a first order correction is obtained by removing the one axis image jitter (Gradient tilt) variance from the long exposure FWHM: Outer scale correction coefficient to apply to the DIMM estimates of the image quality of a 8m telescope limited by the atmosphere, for 0 and 60 degree zenith angle, as a function of the observing wavelength (the following central wavelength of the bands [U, B, V, R, I, J, H, K, L, M, N] corresponding to [0.36, 0.44, 0.55, 0.64, 0.79, 1.25, 1.65, 2.2, 3.4, 5.0, 10] in mm). Zanjan, Iran

  23. Monitoring Turbulence Height with the DIMM Scintillation through DIMM apertures of 10-12cm diameter can be related to the isoplanatic angle (Loos & Hogge, Appl. Opt. 18, 15; 1979) and then to the normalized 5/3rd moment of the turbulence height (Hbar). The atmospheric seeing (black lower curve, in arcsec) is the cumulative effect of several turbulent layers at various altitudes: monitoring the characteristic altitude of the turbulence (red upper curve, in km) is necessary for planning adaptive optics instrumentation. In this example, the bad seeing is located at low altitude while good conditions are produced by a few layers at high altitude. Zanjan, Iran

  24. Local Seeing: Ground Layer Turbulence at Paranal Measurement of the microthermal activity and Seeing at Paranal (GSM Campaign, Nice University) during a night presenting variable conditions (F. Martin, R. Conan, A. Tokovinin, A. Ziad, H. Trinquet, J. Borgnino, A. Agabi and M. Sarazin; Optical parameter relevant for high angular resolution at Paranal from GSM instrument and surface layer contribution; Astron. Astrophys. Supplement, v.144, p.39-44; June 2000). Zanjan, Iran

  25. Local Seeing: Seeing Impact of Ground Layer Measurement of the microthermal activity and Seeing at Paranal (GSM Campaign, Nice University): The contribution of the layer 7-21m above ground is marginal both during good and bad seeing conditions . Zanjan, Iran

  26. Conclusion Intercalibration of the site monitoring instruments is recommended Zanjan, Iran

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